It is necessary to examine for material defects the internal structure of numerous solid and partially solid products as well as intermediate products. To this end, there is a need for nondestructive test methods that provide information relating to the internal, invisible structure. This is required for mechanically highly stressed components, in particular.
For example, components made from steel are forged after being cast, and are subsequently brought into the final shape by turning. In this process, the testing for internal material defects can be performed as early as after the forging.
Such metal parts are usually tested with ultrasound. Sound waves that are reflected at interfaces in the material are detected in this case. The travel time of the reflected sound wave can be used to determine the path length it has covered. Further information relating to the material defect or defects can be obtained by insonification from various directions. Material defects can be located therefrom, by way of example. The geometric alignment of the material defect can be determined in this way, for example. Conclusions on the type of the material defect can be drawn from the shape of the reflected sound waves.
The volume accessible to the ultrasound can be completely investigated by scanning the surface of the test object with an ultrasonic detector and recording the data detected. An image can be generated from the detected data and be used for evaluation.
There are a number of options for determining the size of material defects. For example, the extent of the material defect can be read off directly during scanning. However, this requires the spatial resolution being smaller than the spatial extent of the material defect. The spatial resolution is limited by the wavelength used and the size of the aperture and therefore also by the diffraction of the sound waves.
The size of the material defect can also be determined with the aid of the amplitude of the reflected signal. It is thereby also possible to determine the size of such material defects as are smaller than the spatial resolution of the method. However, the amplitude of the reflected signal is also a function of further parameters, for example the orientation of the material defect, or the reflection properties at the interface.
The amplitude of the reflected signal decreases with decreasing size of the material defect. In this case, the spacing of the interference signals becomes too small to be able to identify the material defect from a single amplitude/travel time diagram. A spacing of +6 dB is expediently required between the measurement signal and the interference signal.
The spatial resolution can be optimized by focusing the sound waves with the aid of suitable test heads. In this process, the focusing can become narrower the wider the test head in relation to the wavelength. The focusing effects a higher sound pressure.
FIG. 4 shows a schematic sectional view of a test object 10 with a material defect 30. Located on the outside of the test object 10 is a test head 16 that is designed as a focusing test head. The test head 16 emits focused sound waves 32, 34 and 36. Here, the continuous line represents the wave front of the current sound wave 32. The dashed lines represent the wave fronts of the earlier sound waves 34 and the later sound waves 36. The focused sound waves 32, 34 and 36 propagate along a predetermined direction with a laterally bounded extent.
During scanning, the test head 16 moves on the surface of the test object 10 along a scanning direction 38. However, the focusing occurs only inside the near field of the test head 16. The larger the width of the test head 16 perpendicular to the scanning direction, the greater can be the distance of the detectable material defect 30.
One possibility for assessing the material defects is to evaluate the amplitude using the spacing/gain/size method (AVG method). Starting from the amplitude, the material defect is assigned an equivalent reflector size that would produce a vertically insonified free circular surface. When the detected signal is substantially greater than the interference signal or noise signal, there is no problem in evaluating the amplitude using the AVG method. In this case, the reflector must be located on the acoustic axis of the sound field of the test head 16. From the dependence of the amplitude on the spacing from the test head 16, the detected amplitude corresponds to a reflector size with known geometry and orientation relative to the acoustic axis. If, by contrast, the detected amplitude is smaller than the noise signal or of a comparable order of magnitude, the material defect cannot be identified from the amplitude/travel time diagram.
Another method for improving the spatial resolution is the synthetic aperture focus technique (SAFT), in which use is made of a small, nonfocusing test head. Here, a three-dimensional image of the test object is calculated with the aid of a two-dimensional mechanical scanning of the test object.
A schematic sectional view of a test object 10 with a material defect 30 is illustrated in FIG. 5 for the purpose of explaining the SAFT method. The test head 16 is located on the outside of the test object 10. By comparison with FIG. 4, the test head has a relatively small diameter and is not of focusing design. Spherical sound waves 42, 44 and 46 are emitted by the test head 16. The wave front of the current spherical sound wave 42 is illustrated by a continuous line. The dashed lines represent the wave fronts of the earlier spherical sound waves 44 and of the later spherical sound waves 46. A comparison of FIG. 4 and FIG. 5 makes clear that the wave fronts 32, 34 and 36 of the focused sound waves, on the one hand and the wave fronts 42, 44 and 46 of the spherical sound waves, on the other hand, are oppositely curved.
In the case of the SAFT method, the test object 10 is subdivided into volume elements by a computer. Each volume element is successively regarded as a reflector during scanning. The reflected signal components from various positions of the test head 16 which belong to the same volume element are recorded and added up in phase with the aid of the computer. In this way, echo signals of large amplitude are obtained on the basis of constructive interference only for such locations as have actual reflection. The echo signals are extinguished on the basis of destructive interference for locations without actual reflection. In the case of constructive interference, the scanning and computing operation simulates an ultrasonic detector whose size corresponds to the scanned surface and which is focused onto a location.
It is possible to determine therefrom the position of the material defect and, in the event of an extended material defect, also to determine the size thereof within the scope of the resolution. The accuracy is approximately comparable to that in the scanned region in the case of the above-named method, which uses the focused sound waves. With the SAFT method, the spatial resolution is not limited by the dimensions of the test head 16, and so a high spatial resolution is possible.
With the SAFT method, all the reflected signal components coming into consideration in each pixel in the expected defect region are added with a time shift that the signal components would have if the pixel were the source of a reflected wave. The time shift that corresponds to the phase angle results from the geometric relationships between the test head 16 and the pixel, particularly from the spacing between the test head 16 and the pixel. If the pixel is now actually the source of a reflected wave, the amplitude then increases at this site with the number of the various positions of the test head 16 from which the material defect was detected. For all other pixels, the phases do not correspond, and so the sum vanishes in an ideal case, but is at least very small.
The SAFT method is mostly used in order to achieve a high spatial resolution. In principle, this is a focusing method in which the limit of resolution results from the wavelength and the synthetic aperture. The synthetic aperture is determined by the angular range from which the material defect is detected. The aperture is limited by the movement of the test head 16 and the divergence of the sound field.
The test object can, by way of example, be a rotor of a gas or steam turbine that is used, in particular, for power generation. Such a rotor is exposed to high stress during operation. The speed of the rotor corresponds to the line frequency of the respective network. For example, a speed of 3000 revolutions per minute is required for a network with a line frequency of 50 Hz. Large centrifugal forces occur on the rotor in the case of such high speeds. The centrifugal forces increase with the diameter of the rotor. The larger the turbine is designed, the stronger also are the centrifugal forces.
When the turbine is started, the rotors are, in particular, strongly loaded thermally in a tangential direction. In this phase, the rotor is firstly cold and is brought up to operating temperature from the outside inward by the hot combustion gases. Consequently, the number of the starts is of particular significance for the lifetime of the turbine. The tangential loading is greatest for the rotor in the region of its central bore. Consequently, material defects in the vicinity of the bore have a decisive influence on the longevity of the turbines. Particularly in the coming generation of turbine wheel disks, there is a need for a clear increase in the detection sensitivity for axially/radially oriented material defects. A sufficiently accurate determination of the axially/radially oriented material defects is impossible with the test methods to date.
Because of the higher powers of the recent gas or steam turbines, there is a rise in requirements for the rotor to be free from material defects. The size of the rotors is also increasing, and this entails longer ultrasonic paths in the case of material testing. The minimum value of the detectable material defects increases in the inner region of the rotor owing to the larger path length of the ultrasound. There is thus a need for a method that can also enable material defects to be determined in large components.